Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Transmission Modes
A user equipment is configured with a transmission mode to help the user equipment to determine how to process data transmissions received on a Physical Downlink Shared Channel (PDSCH). In LTE release 8, a total of seven transmission modes were defined. Transmission modes used in Frequency Division Duplex (FDD) mode of operation have been designed to use Common Reference Symbols (CRS) for data demodulation. CRS may be cell-specific reference signals. The PDSCH demodulation reference signals used in LTE may be CRS, Cell-specific reference signals, and UE-specific reference signals.
Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting PDSCH transmission, at least before LTE release 12. Demodulation Reference Signals (DM-RS) are transmitted only on the resource blocks upon which the corresponding PDSCH is scheduled to the user equipment. DM-RS based transmission modes are e.g. Transmission Mode 7 (TM7) defined in LTE release 8, Transmission Mode 8 (TM8) defined in LTE Release 9, Transmission Mode 9 (TM9) defined in LTE Release 10, Transmission Mode 10 (TM10) defined in LTE Release 11, Transmission Mode x (TMx) defined in LTE Release 12. Transmission Modes 1, 2, 3, 4, 5, 6 (TM1, 2, 3, 4, 5, 6) are transmission modes with CRS as the demodulation reference signal.
The DM-RS are transmitted relatively dense in a LTE resource element grid as long as a user equipment is scheduled for DownLink (DL) data. FIG. 1 shows a resource element grid of an LTE subframe where DMRS are embedded. The X axis represents the time domain, number is the OFDM symbol index within one subframe, 1 ms. The Y axis represents the frequency domain, number is the subcarrier index within one Resource Block.
A synchronized network is a network wherein all cells have aligned radio frame and subframe boundary. When a Transmission Mode with DM-RS is deployed in a synchronized network with unshifted CRS configured, DMRS are not interfered by CRS which results in an improved demodulation and receiver performance compared to Transmission Modes which use CRS for demodulation, which CRS are interfered. Transmission Mode with DM-RS therefore can enjoy very good conditions for data demodulation in this scenario.
According to US20120087321, a transmission scheme adds a DM-RS to estimate a channel response for demodulation. When a transmission scheme based on LTE CRS is set in a user equipment, the user equipment cannot receive data in a Multimedia Broadcast Multicast Service Single Frequency Network (MBSFN) sub-frame. To solve this problem a present scheme disclosed in US20120087321 separates and sets a transmission mode for sub-frame and a transmission mode for MBSFN for the user equipment. The user equipment uses different transmission modes in different subframes.
CRS is not transmitted in data region of an MBSFM subframe, the user equipment with CRS based transmission mode cannot receive unicast PDSCH data in MBSFN. This document discloses to use a DM-RS based transmission mode in MBSFN subframe or use the CRS from MBSFN subframe control region to demodulate the unicast PDSCH data.
WO2012129798 discloses a method for applying open-loop pre-coding corresponding to TM3, but over TM9 in because it has not been defined in the LTE standard release. The document discloses how TM9 is adapted more specifically with choice of precoder to Multiple Input Multiple Output (MIMO) rank, velocity of user equipment, antennas etc. The document further discloses how DM-RS is applied depending on rank.
DM-RS will add more overhead leaving less symbols available for data. Extra overhead percentage depends on different control region size, transmission mode, number of antenna ports, etc. The extra overhead percentage may e.g. be ˜10%-20%.
To configure a network for a Transmission Mode with DM-RS only, will in most cases result in a not so good capacity because of larger overhead.
To configure for a Transmission Mode with DM-RS transmission per cell is time consuming and costly requiring advanced measurements and network planning.